The group III nitride semiconductor expressed by AlaGa1-aN (0≤a≤1) can regulate (modify) the composition of the group III element (aluminum (Al), gallium (Ga)), thereby the luminescence peak wavelength can be selected arbitrarily within the range of 210 to 365 nm; the band structure with a direct bandgap type is attained within the energy range corresponding to the above mentioned wavelength range, thus the group III nitride semiconductor is a most suitable material for forming ultraviolet ray emitting element.
The ultraviolet ray emitting element made of the group III nitride semiconductor is generally produced by carrying out a crystal growth of an AlaGa1-aN layer on the single crystal substrate by a metal organic chemical vapor deposition method (MOCVD method) and molecular beam epitaxy method (MBE method). As the single crystal substrate, preferably the material having good matching lattice constant and matching thermal expansion coefficient with AlaGa1-aN layer is used.
For example, non-patent articles 1 and 2 obtain the ultraviolet ray emitting element having the luminescence wavelength of 300 nm or less by growing an AlGaN layer using a sapphire as the substrate. The non-patent articles 1 and 2 disclose that the luminescence efficiency and the element lifetime are improved by reducing the dislocation density of the AlaGa1-aN layer.
Also, in case of growing the AlaGa1-aN layer (0<a≤1) which includes Al, the aluminum nitride (AlN) single crystal layer is used to reduce the defect density in the AlaGa1-aN layer including Al, and as an advantageous means for producing the highly efficient ultraviolet ray emitting element. For example, the patent document 1 discloses that the dislocation density in the AlaGa1-aN layer can be reduced, and the ultraviolet ray emitting diode having a luminescence peak wavelength of 250 nm is obtained by producing the ultraviolet ray emitting diode including a laminate structure comprising the AlaGa1-aN layer which is lattice matched with the AlN single crystal layer on the AlN single crystal substrate.
Also, the patent document 2 discloses the ultraviolet ray emitting element, wherein a cap layer which is substantially lattice relaxed, that is having substantially the same lattice constant as distortion free condition (the condition wherein the lattice relaxation rate is 100%), is provided on a similar structure (on the AlaGa1-aN layer which is lattice matched). This patent document 2 discloses that by lattice relaxing the cap layer, the doping can be easily done, and the cap layer can be thickened, and also because a foundation layer is not influenced by the distortion of the cap layer, effects such as thickening of the AlaGa1-aN layer as the foundation layer can be expected. Further, a p-type gallium nitride (GaN) layer, which is the cap layer thereof, has large difference of the lattice constant between the AlaGa1-aN layer which is lattice matched with the AlN single crystal substrate, thus it discloses that this will grow on the AlaGa1-aN layer by Stranski-Krastanov(SK) mode or Volmer-Weber(VW) mode. Due to this growth, during the initial growth stage, GaN is known to undergo three dimensional crystal growth such as forming an island.
Also, the non-patent document 3 discloses that in case the GaN layer is 100% lattice relaxed (in case the lattice relaxation rate of the GaN layer is 100%) wherein the GaN layer is grown on AlN layer formed on the sapphire substrate, then the island form crystalline having a height of 500 nm or so is formed which corresponds to 2 times or so of the designed value. Further, by using the nitrogen carrier gas during the growth of the GaN layer, the smoothness of the GaN layer is improved, and also the lattice relaxation rate of the GaN layer thereof was 75%. Also, the patent document 3 discloses the example of using nitrogen gas when growing GaN layer, wherein the ratio of nitrogen carrier gas is 0.5 or more, and the example shows the carrier gas ratio having 0.9 of nitrogen gas and 0.1 of hydrogen gas.